This invention relates to a magnetic recording medium such as a magnetic tape and a process therefor and, more particularly, to the improvement of its back coat layer.
Magnetic recording media such as magnetic tapes (hereinafter referred to as "tapes") have a structure wherein a transparent non-magnetic substrate such as polyester is provided with a magnetic layer containing fines of magnetic substance and a binder resin. The non-magnetic substrate has generally electrical insulating property and therefore drawbacks such as drop-out and the reduction in runnability can occur due to foreign matter deposition by charging. The influences of such a charging are reduced by adding a conductive material such as carbon black to the magnetic layer.
There are many apparatus utilizing the difference between the transmitted light in tapes and the transmitted light in leader tapes as mechanisms for detecting the terminal of tapes. In order to successfully carry out such a terminal detection, it is necessary to reduce the light transmittance of the tapes. Accordingly, materials such as carbon black are added to the magnetic layer.
Recently, magnetic recording media having a high density have been increasingly requested. However, the non-magnetic materials such as carbon black described above reduce the magnetic flux density in the magnetic layer and interfere with the high densification of magnetic recording.
An attempt has been carried out to reduce the amount of the non-magnetic material in the magnetic layer by adding such non-magnetic materials to a back coat layer. In this case, a binder resin and a non-magnetic material such as carbon black are contained in the back coat layer. Such a back coat layer comes into contact with a number of guides during the tape running process and therefore high durability is requested. In order to meet such a requirement, the binder resin in the back coat layer is cured. Curing means include thermosetting, electron beam curing and photosetting by ultraviolet light.
However, these curing means pose the following drawbacks. First, thermosetting generally takes a long period of time and therefore the binder resin is cured after the back coat layer is applied to the non-magnetic substrate and thereafter wound in the form of a roll. Accordingly, the back coat layer comes in contact with non-magnetic substrate, whereby the back coat layer is affected by the surface of the non-magnetic substrate.
In electron beam curing, the drawbacks as described in the thermosetting process do not occur. However, the apparatus used is complicated and productivity is low. Further, in the case of photosetting, the back coat layer has a low permeability to a light and therefore the curing of the binder resin does not sufficiently proceed.